DESCRIPTION: (Applicant's Description) Appropriate control selection is often the most important determinant of the quality of an epidemiologic study. For our studies of iatrogenic causes of cancer in Los Angeles, for many years we have chosen to use neighborhood controls selected by following a predetermined algorithm which proceeds through an obligatory sequence of adjacent houses and residential units, beginning at a specific residence which has a specific geographic relationship to the residence where the case lived at diagnosis. We have developed a detailed manual of procedures for this resource. The timeliness and accuracy of the field representatives who conduct this neighborhood canvassing, are monitored and evaluated through a series of quality control procedures, including rewalks, as well as regular reports of activities to the Field Supervisor of this Resource, the Core Leader, and to individual project leaders or study managers. Using this strategy we routinely identify and interview controls for over 95 percent of interviewed cases. Refusal rates are generally at or below 20 percent.